Voltage charge pump with segmented boost capacitors

ABSTRACT

A voltage charge pump circuit with boost capacitor segments and boost delay chain structures are provided. The voltage charge pump circuit comprising a plurality of boost capacitor segments each of which is individually controlled by a respective signal line of a boost delay chain structure.

FIELD OF THE INVENTION

The invention relates to a voltage charge pump circuit and, moreparticularly, to a voltage charge pump circuit with boost capacitorsegments and boost delay chain structures.

BACKGROUND

Phase change memory (PCM) is an emerging segment of semiconductor memorytechnology. Phase change memory operation requires a variety of DC powersupply voltages to support read and write operations, with some voltagesbeing higher than the external power supply. For example, phase changememory operation requires a variety of voltages to be generated on dieto support operation with good DC characteristics of up to μS duration,e.g., ˜0.4 V for bitline precharge, ˜1 V for standard logic and ˜2.5 Vfor wordline, read sense, and write supply. Tight power supplytolerances are required for resistance sensing and writing PCM arrayswith differing load currents and duration.

Voltage charge pump circuits are required to raise the voltages higherthan the external power supply. The requirements/demands on the highvoltage supply are challenging and include the requirement of low ripplewhile being able to supply current over a wide range, high currentsupply capability, and good tolerance. However, voltage charge pumpsystems developed from other types of memory devices do not serve theunique requirements of phase change memory sufficiently. In other words,they do not supply high voltage over a wide range, with high currentcapability while also having low ripple.

Charge pumps are used in integrated circuits to provide a boosted supplyvoltage in applications such as eDRAM memory, FLASH memory, bandgapvoltage references, etc. Typical boost circuits first charge a capacitorfrom an external supply and then transfer the stored charge into acapacitor on a boosted supply net. A ripple voltage, though, develops onthe boosted supply net that needs to be minimized. Currently, theseripples are minimized by placing large filter capacitors (also calleddecoupling capacitance) on the boosted supply net and by the use ofmultiphase pumps.

For a given design, a charge pump supplies current to drive an intendedload. The output load current can vary greatly depending upon thedifferent operating modes or other conditions. Further, the pump outputcurrent will increase with higher pump frequency and supply voltage. Toregulate the output voltage of a charge pump with good precision and atight voltage tolerance it is necessary to use a regulator circuit tocontrol a charge pump, typically turning it on and off so at to keep theoutput voltage within a desired range. Typically, a charge pump providessome excess charge before it can be turned off. In addition, the boostedvoltage must fall to a lower potential before a regulator will turn thepump back on. The excess charge and fall in potential is acharacteristic of regulation and adds to voltage ripple on the boostedsupply net.

More specifically, a charge pump system can be sized to provide adequatecurrent at low supply voltage. However, it will also typically haveexcessive ripple voltage unless the dcap is sized for the higher chargetransfer at the maximum supply voltage. But, decoupling capacitancetypically consumes close to 50% of the total pump area in the design andhas a significant impact on chip size.

SUMMARY

In an aspect of the invention, a voltage charge pump circuit comprises aplurality of boost capacitor segments each of which is individuallycontrolled by a respective signal line of a boost delay structure

In an aspect of the invention, a charge pump architecture comprises aplurality of pump boost capacitor segments enabled as a function ofresponse of power to current load.

In an aspect of the invention, a method comprises enabling of individualboost capacitor segments coordinated with delays to reduce forward biasand ripple.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The present invention is described in the detailed description whichfollows, in reference to the noted plurality of drawings by way ofnon-limiting examples of exemplary embodiments of the present invention.

FIG. 1 shows a voltage charge pump circuit with boost capacitor segmentsand a boost delay chain structure in accordance with aspects of thepresent invention.

FIGS. 2a and 2b show boost delay chain structures used with a voltagecharge pump circuit of FIG. 1, in accordance with aspects of the presentinvention.

FIG. 3 shows a boost capacitor waveform comparing a voltage charge pumpcircuit with a single boost capacitor vs. boost capacitor segments witha long delay.

FIG. 4 shows a boost capacitor waveform comparing a voltage charge pumpcircuit with a single boost capacitor vs. boost capacitor segments witha short delay.

DETAILED DESCRIPTION

The invention relates to a voltage charge pump circuit and, moreparticularly, to a voltage charge pump circuit with boost capacitorsegments and boost delay chain structures. More specifically, thepresent invention comprises a PFET voltage charge pump circuit withboost capacitor segments with individual boost timing control ofcapacitor segments during a boost phase. Advantageously, the voltagecharge pump circuit can reduce parasitic pump currents and improve pumpefficiency, while providing an efficient way to manage boost nodeforward bias for faster pump cycle times without forward bias chargeinjection. In addition, the voltage charge pump circuit can be mappedeasily into different technologies.

In embodiments, the voltage charge pump circuit partitions the boostcapacitor portion of a voltage pump circuit into boost capacitorsegments. The activation of the boost capacitor segments can bestaggered in time to manage forward bias of source and drain junction ofthe PFET based voltage charge pump circuit and can also be individuallyenabled to allow system level adjustability of pump output current. Theadjustability of output current permits the pump output to be regulatedfor current demand to reduce ripple on the supply.

In more specific embodiments, the voltage charge pump circuit comprisesa voltage pump which is divided into multiple smaller capacitances witheach of the smaller capacitances (boost capacitors) having a separatecontrol timing so each of the multiple smaller capacitances can fire ata different time. The timing of each smaller capacitance can beseparately controlled to limit the over voltage of the boosted node. Forexample, the delay of timing can be controlled, individually, making itpossible to adjust the delay of the overvoltage produced on the boostednode. In further embodiments, the individual boost capacitor segmentscan be coordinated with delays, e.g., controlled by digital enablesignals, to control whether the boost capacitor segment fires at all. Inembodiments, it is also possible to control the enable to control outputcurrent of the charge pump and to control the ripple of the charge pump.In embodiments, the signal delays can be created with analog circuittypes and using analog circuit methods.

The voltage charge pump circuit described herein can be manufactured ina number of ways using a number of different tools. In general, though,the methodologies and tools are used to form structures with dimensionsin the micrometer and nanometer scale. The methodologies, i.e.,technologies, employed to manufacture the voltage charge pump circuithave been adopted from integrated circuit (IC) technology. For example,the voltage charge pump circuits are built on wafers and are realized infilms of material patterned by photolithographic processes on the top ofa wafer. In particular, the fabrication of the voltage charge pumpcircuit uses three basic building blocks: (i) deposition of thin filmsof material on a substrate, (ii) applying a patterned mask on top of thefilms by photolithographic imaging, and (iii) etching the filmsselectively to the mask.

In further aspects of the invention, the method comprises enabling ofindividual boost capacitor segments coordinated with digital delays tocompensate for a higher pump frequency and/or supply voltage. Moreover,the method comprises enabling of individual boost capacitor segmentscoordinated with digital delays to compensate for current loads thatvary due to different modes or other conditions.

FIG. 1 shows a voltage charge pump circuit with boost capacitor segmentsand a boost delay chain structure in accordance with aspects of thepresent invention. In embodiments, the circuit pump system 10 includes aplurality of boost capacitor segments 12, 14, 16 and 18 with associateddrive circuitry as described further herein. Although four boostcapacitor segments are shown in the circuit pump system 10, one of skillin the art would appreciate that any number of boost capacitor segmentsand associated drive circuitry can be implemented within the scope ofthe present invention.

In embodiments, the boost capacitor segments 12, 14, 16 and 18 eachinclude, for example, an NFET 20 configured to be used as the capacitorof the segmented boost capacitor. The boost capacitor segments 12, 14,16 and 18 are driven by boost devices, e.g., boost transistors. In morespecific embodiments,

(i) boost capacitor segment 12 includes boost transistor T23 (T23 is aNFET transistor configured to be used as the capacitor of the boostcapacitor segment 12) with associated boost drive devices (transistorsT12, T17-T19);

(ii) boost capacitor segment 14 includes boost transistor T26 (T26 is aNFET transistor configured to be used as the capacitor of the boostcapacitor segment 14) with associated boost drive devices (transistorsT28, T29-T31);

(iii) boost capacitor segment 16 includes boost transistor T27 (T27 is aNFET transistor configured to be used as the capacitor of the boostcapacitor segment 16) with associated boost drive devices (transistorsT33, T32, 34 and 35); and

(iv) boost capacitor segment 18 includes boost transistor T37 (T37 is aNFET transistor configured to be used as the capacitor of the boostcapacitor segment 18) with boost drive devices (transistors T39, T36, 38and 40).

In embodiments, the boost capacitor segments 12, 14, 16 and 18 arechained together by a boost delay chain 22, comprising a pair ofinverters 24 (in series) on each signal line designated by its inputboost signals, CN, CNd1, CNd2, CNd3. As should be understood by those ofordinary skill in the art, the pair of inverters 24 (in series) willpreserve the polarity of the signal. The output of each of the boostcapacitor segments 12, 14, 16 and 18 are connected to node R5, whichfeeds to an output PFET 16.

By using the delayed signals generated by the boost delay chain 22, eachboost capacitor segment 12, 14, 16, 18 can fire at a different time,sequentially, thus feeding a separate boost signal to output PFET 16.Thus, the timing of each smaller capacitance 12, 14, 16 and 18 can beseparately controlled to limit the over voltage of the boosted node. Byway of example, the pair of inverters 24 of the boost delay chain 22will provide a delayed boost signal, e.g., CNd1, CNd2, CNd3, to eachsubsequent segmented boost capacitor 14, 16 and 18 as follows:

(i) initial input boost signal CN for controlling boost capacitorsegment 12 is input to the pair of inverters, which results in a delayedoutput signal CNd1;

(ii) delayed boost signal CNd1 then controls boost capacitor segment 14,which is also input to the pair of inverters resulting in a delayedoutput signal CNd2;

(iii) delayed boost signal CNd2 then controls boost capacitor segment16, which is also input to the pair of inverters resulting in a delayedoutput signal CNd3; and

(iii) delayed boost signal CNd3 then controls boost capacitor segment18.

FIG. 2a shows an alternate boost delay chain structure with individualsegment enables, in accordance with aspects of the present invention.More specifically, the boost delay chain structure 22′ of FIG. 2aincludes individual enable signals for controlling each boost capacitorsegment 12, 14, 16, 18 (e.g., boost transistor and boost drive devices).In embodiments, individual signal lines for each of the boost capacitorsegments includes an AND 28 gate in series with a non-inverting buffer30, which maintains the polarity of the input signals.

In operation, the individual boost capacitor segments 12, 14, 16, 18 ofFIG. 1, for example, can be coordinated with digital delays (enablesignal), e.g., enable0, enable1, enable2, enable3. By using the enablesignals as inputs to each NAND gate 28, it is now possible to controlhow many of the boost capacitor segments will fire. For example, any ofthe enable signals not activated for a signal line in the boost delaychain 22′, the associated boost signal, e.g., CNd0, CNd1, CNd2, CNd3,and all subsequent boost signals in the chain will not fire for theassociated boost capacitor segment 12, 14, 16 or 18.

FIG. 2b shows another embodiment of boost delay chain with individualsegment enables such that disabling any segment does not break the delaychain. Subsequent boost segments beyond a disabled segment still fire ifenabled.

In embodiments, the enable signal can be activated using load sensing(e.g., comparator) to allow better regulation of boost voltage vs. loadcurrent. That is, the charge pump architecture can control a number ofpump boost capacitor segments to be enabled as a function of theresponse of power system to current load.

More specifically, upon activation of each enable signal, in sequence,

(i) input signal CN will control boost capacitor segment 12, which isalso input to the NAND gate and inverter, resulting in a delayed outputsignal CNd0;

(ii) delayed signal CN_d1 then controls boost capacitor segment 14,which is input to the NAND gate and inverter resulting in a delayedoutput signal CNd1;

(iii) delayed signal CN_d2 then controls boost capacitor segment 16,which is also input to the NAND gate and inverter resulting in a delayedoutput signal CNd2; and

(iii) delayed signal CN_d3 then controls boost capacitor segment 18.

In this way, it is possible to sequentially activate and control each ofthe boost capacitor segments, individually, by activating the enablesignal. This provides the ability to control individual segments, insequence, in order to proportion the output current, e.g., adjust orfine tune the output current during different modes, e.g., standby mode.This, in turn, allows the output current to be fine tuned,incrementally, and reduce any excessive ripple in proportion to the loadcurrent output from the voltage pump.

By way of an example illustration, Table 1 shows a comparison of rippleat different load currents for a conventional charge pump system andthat which is described with respect to FIG. 1. Table 1 showsimprovement of ripple using the voltage charge pump circuit of theinvention.

TABLE 1 Design (mA) Idle Stby 3.2 4 4.8 5.6 6.4 7.2 8 8.8 (100 μA) (100μA) Conventional 26.6 21.8 24.4 24.1 24.1 22.5 21.2 19.6 22.3 15.1 PumpSegmented 9.6 9.6 9.3 8.7 9.7 10 10.1 8.8 11.7 10.6 charge pump

FIG. 3 shows a boost capacitor waveform comparing a voltage charge pumpcircuit with a single boost capacitor vs. boost capacitor segments witha long delay in accordance with aspects of the invention. As shown inFIG. 3, the single boost capacitor has a peak charge of about 2.9 V,compared to the voltage charge pump circuit with the segmented boostcapacitors which have a peak voltage of about 2.6 V for each of theinput signals, CNd0, CNd1, CNd2, CNd3.

The voltage spike of the single boost capacitor results in a largeforward bias at node R5, which is not well tolerated. For example, withincreasing forward-bias voltage, the depletion zone of the output PFET16 at node R5 eventually becomes thin enough that the zone's electricfield cannot counteract charge carrier motion across the p-n junction,as a consequence reducing electrical resistance. The electrons thatcross the p-n junction into the P-type material (or holes that crossinto the N-type material) will thus diffuse in the near-neutral region.Therefore, the amount of minority diffusion in the near-neutral zonesdetermines the amount of current that may flow through the diode. Also,this voltage spike will result in transistor latch-up. In comparison,the forward bias resulting from the peak voltage of about 2.6 V usingthe segmented boost capacitors is well tolerated, thus avoiding latch-upissues and providing a smoother output on node R5.

FIG. 4 shows a boost capacitor waveform comparing a voltage charge pumpcircuit with a single boost capacitor vs. boost capacitor segments witha short delay. As shown in this representation, at about 9.962milliseconds, the ripple will begin to blend together using the voltagecharge pump circuit with the segmented boost capacitors. This is incomparison to the single boost capacitor.

Operation of Voltage Charge Pump Circuit

With Boost Capacitor Segments

As each of the boost capacitor segment and related circuitry comprisesimilar structures, only an explanation of operation of the segmentedboost capacitor 12 is required for an understanding of the presentinvention. By way of explanation of the operation of the circuit pumpsystem 10, several power supply connections are provided, including aground potential connection which is typically 0 Volts, a high potentialsupply which in this example is 1.8 Volts and a VHX which is a prechargevoltage supply equal to the high potential or it may be at a differentpotential. VHY is the output of the pump, which has a voltage level thatis boosted higher than the VHX supply potential by operation of thecircuit pump system 10.

For segmented boost capacitor 12, inverter INV7 and transistors T20-T22,as well as inverter INV10 and transistors T3, T5, T10 and T11 comprise aprecharge circuit which is used to charge the boost transistor T23 gatenode to a VHX potential during the precharge phase. The inverter INV9and transistors T2, T15 and T16, and inverter INV8 and transistors T4,T6, T8 and T9 comprise the output circuit which allows the boost chargeto flow from circuit node R5 to the output of the charge pump VHY duringthe boost phase.

As should be understood, each segment will have two phases in operation.A first phase of operation is the precharge phase. During this phase,for example, the circuit node R2A (for boost capacitor segment 12) isheld to ground potential by transistor T17 and signal FN is at a highpotential. Circuit node R1A is held at a high potential via PFETtransistor T18 with NFET transistor T19 turned off. At the same timecircuit output node R5 is being precharged to the VHX potential by PFETtransistor T21, whose gate, circuit node R3, is held at a groundpotential by the output of inverter INV7 through transistor T22. Theduration of the precharge phase is such that node R5 and the gate of theboost transistor T23 will substantially reach a full VHX potentialbefore the end of the precharge phase.

At the end of the precharge phase signals BN and FN are at a highpotential holding circuit nodes R3 and R9 to a ground potential at theoutputs of inverters INV7 and INV8. Also at the end of the prechargephase signals AN and CN are at a ground potential, circuit nodes R4 andR6 at the boosted supply potential, VHY, transistors T3 and T15 aresubstantially cut-off and not conducting current. Because node R6 is ata boosted high potential, circuit node R8 is held at ground potentialthrough NFET transistors T6 and T4. Because circuit node R8 is at groundpotential, PFET transistor T2 is on and holds circuit node R4 to theboosted output potential of VHY and output transistor T16 is cut-off andnot conducting current.

The second phase of operation is the boost phase of each segmented boostcapacitor. This phase directly follows the precharge phase. At the startof the boost phase, signal BN transitions to a ground potential causingthe output of inverter INV8 to transition to a high potential andraising the potential of the node R9 to a NFET threshold below the highpotential through NFET transistor T4. Also FN transitions to a groundpotential turning off NFET transistor T17 and causing output of inverterINV7 to transition to high potential bringing circuit node R3 and thegate of PFET transistor T21 to a NFET threshold below the high potentialthrough NFET transistor T22.

At the next step of the boost phase signal AN transitions to a highpotential causing the output of inverter INV10 to transition to a groundpotential and pulling circuit nodes R6 through transistor T3 and node R7to ground potential through NFET transistor T5. With node R6 at a groundpotential, circuit node R8 is pulled up to the boost potential VHY viaPFET transistor T8 turning off PFET transistor T2. Also with R6 atground potential R9 is pulled up to the boosted potential VHY via PFETT9. Further with node R7 at a ground potential PFET transistor T20 isturned on strongly connecting circuit node R3 to circuit node R5. Thecapacitance of circuit node R5 is much greater than circuit node R3causing R3 to substantially charge up to and become equal to the R5 highpotential.

The last step of the boost phase, signal CN transitions to a highpotential causing circuit node R1A to discharge to a ground potentialthrough NFET transistor T19 which in turn causes PFET transistor T12 toturn on and raise circuit node R2 and the source and drain nodes of nettransistor T23, the boost transistor to a high potential and pumpingcharge onto circuit node R5. Signal CN rising also causes the output ofinverter INV 9 to transition to ground potential turning on PFET outputtransistor T16. With output transistor T16 turned on the charge pumpedonto boost node R5 flows onto the boosted output VHY.

The boost phase ends and the next precharge phase begins with signal ANtransitioning back to a ground potential which will bring circuit nodesR6 and R7 to a threshold below the high potential followed by CNtransitioning back to a ground potential which causes node R1A totransition to a high potential turning off PFET transistor T12 andraising node R4 to a threshold below the high potential. After thetransition of CN, BN rises to a high potential and returns nodes R8 andR9 to ground potential. Node R8 at ground potential turns on transistorT2 and further raises node R4 to the boosted supply level fully turningoff PFET output transistor T16. The signal FN can then rise to a highpotential as the start of the next precharge phase. With FN at a highpotential, circuit nodes R2A and R3 are returned to ground potential andthe gate of NFET boost capacitor T23 begins precharging to the VHXpotential again.

The method(s) as described above is used in the fabrication ofintegrated circuit chips. The resulting integrated circuit chips can bedistributed by the fabricator in raw wafer form (that is, as a singlewafer that has multiple unpackaged chips), as a bare die, or in apackaged form. In the latter case the chip is mounted in a single chippackage (such as a plastic carrier, with leads that are affixed to amotherboard or other higher level carrier) or in a multichip package(such as a ceramic carrier that has either or both surfaceinterconnections or buried interconnections). In any case the chip isthen integrated with other chips, discrete circuit elements, and/orother signal processing devices as part of either (a) an intermediateproduct, such as a motherboard, or (b) an end product. The end productcan be any product that includes integrated circuit chips, ranging fromtoys and other low-end applications to advanced computer products havinga display, a keyboard or other input device, and a central processor.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

What is claimed:
 1. A voltage charge pump circuit comprising: aplurality of boost capacitor segments each of which is individuallycontrolled by a respective signal line of a boost delay structure,wherein a first signal line of the boost capacitor segments comprises aNAND gate in series with a first inverter, and subsequent signal linescomprise a pair of inverters in series with a single input and a secondNAND gate in series with a second inverter for corresponding boostcapacitor segments.
 2. The voltage charge pump circuit of claim 1,wherein the boost delay structure is structured to allow individualboost timing control of each of the boost capacitor segments during aboost phase.
 3. The voltage charge pump circuit of claim 2, wherein therespective signal line for each of the plurality of boost capacitorsegments of the boost delay structure provides a delayed input boostsignal to a respective boost capacitor segment of the plurality of boostcapacitor segments.
 4. The voltage charge pump circuit of claim 3,wherein the pair of inverters provide the delayed input boost signal toa subsequent boost capacitor segment of the plurality of boost capacitorsegments.
 5. The voltage charge pump circuit of claim 4, wherein thedelayed input boost signal fires each boost capacitor segment at adifferent time.
 6. The voltage charge pump circuit of claim 4, whereinthe pair of inverters are connected in series between the respectivesignal line for each of the plurality of boost capacitor segments andthe delayed input boost signal to the subsequent boost capacitor segmentof the plurality of boost capacitor segments.
 7. The voltage charge pumpcircuit of claim 2, wherein each boost capacitor segment includes aboost transistor configured to be used as a capacitor, driven by boostdrive transistors.
 8. The voltage charge pump circuit of claim 1,further comprising enable signals for enabling each individual signalline such that, upon activation of an enable signal, a respective one ofthe plurality of boost capacitor segments are individually controlled,and the plurality of boost capacitor segments are driven separately tolimit over voltage of a boosted node.
 9. The voltage charge pump circuitof claim 1, wherein timing control of the plurality of boost capacitorsegments are coordinated with digital delays, boost charge of theplurality of boost capacitor segments are distributed over time tocontrol pump output current, and each of the plurality of boostcapacitor segments further comprise a boost drive transistor.
 10. Thevoltage charge pump circuit of claim 9, wherein the boost drivetransistor is a NFET.
 11. The voltage charge pump circuit of claim 1,wherein the NAND gate receives a respective enable signal as an input,and the second NAND gate includes a first input which is electricallyconnected to the single input of the pair of inverters.
 12. The voltagecharge pump circuit of claim 1, wherein the second NAND gate includes asecond input which is electrically connected to an enable signal, andthe enable signal is activated using a load sensing comparator.
 13. Thevoltage charge pump circuit of claim 12, wherein a corresponding boostcapacitor segment of the boost capacitor segments is activated inresponse to activation of the enable signal.
 14. The voltage charge pumpcircuit of claim 1, wherein a boost capacitor segment, which correspondswith the first signal line, is disabled in response to preventingactivation of an enable signal of the boost capacitor segment, and thecorresponding boost capacitor segments, which correspond with thesubsequent signal lines, are fired in response to activation of theenable signal of the corresponding boost capacitor segments.